A plasma processing apparatus is used in a semiconductor manufacturing process for performing etching or thin film formation. As for a power supply source of the plasma processing apparatus, a RF generator is used. In order to efficiently supply power from the RF generator to the plasma processing apparatus, it is required to match an impedance between the RF generator and the plasma processing apparatus (load). Generally, a matching device is interposed, as a unit for matching an impedance, between the RF generator and the plasma processing apparatus as described in, e.g., Patent Document 1.
FIG. 7 is a functional block diagram of a conventional matching device 100. Referring to FIG. 7, the matching device 100 is interposed between a RF generator 2 and a plasma processing apparatus 3. A plasma is generated by the plasma processing apparatus 3 by supplying a high frequency power outputted from the RF generator 2 to the plasma processing apparatus 3 through the matching device 100. In order to efficiently supply the power from the RF generator 2 to the plasma processing apparatus 3, it is required to match an impedance between the RF generator 2 and the plasma processing apparatus 3. An output impedance of the RF generator 2 is generally 50Ω. Therefore, it is preferable to set an input impedance of the matching device 100 to 50Ω by converting an input impedance of the plasma processing apparatus 3 using the matching device 100.
The input impedance of the plasma processing apparatus 3 varies depending on types, flow rates, pressures and temperatures of gases to be supplied to the plasma processing apparatus 3. Therefore, the matching device 100 needs to perform adaptive matching in response to the temporally varying input impedance of the plasma processing apparatus 3.
The matching device 100 shown in FIG. 7 includes a directional coupler 11 for detecting a travelling wave and a reflected wave, a matching circuit 30 having a matching element for matching an impedance between the RF generator 2 and the plasma processing apparatus 3, and a control unit 120 for controlling a circuit constant of the matching element of the matching circuit 30.
Hereinafter, an operation of the directional coupler 11 will be described. A high frequency power (travelling wave: Pf) travelling from an RFin terminal toward an RFout terminal is detected by the directional coupler 11 and outputted to a FORWARD terminal. A high frequency power (reflected wave: Pr) travelling from the RFout terminal toward the RFin terminal is detected by the directional coupler 11 and outputted to a REFLECT terminal. The high frequency power Pf travelling from the RFin terminal toward the RFout terminal is not detected at the REFLECT terminal, or even if detected, the amount thereof is very small. Similarly, the high frequency power Pr travelling from the RFout terminal toward the RFin terminal is not detected at the FORWARD terminal, or even if detected, the amount thereof is very small.
The travelling wave Pf and the reflected wave Pr detected by the directional coupler 11 are inputted to a reflection coefficient calculation unit 21. A reflection coefficient Γ is defined from an amplitude ratio r of the reflected wave Pr to the travelling wave Pf and a phase difference θ therebetween, as in the following Eq. (1).Γ=r·exp(j·θ)(j: imaginary unit)  Eq. (1)
Therefore, the reflection coefficient Γ can be obtained if the amplitude ratio r of the reflected wave Pr to the travelling wave Pf and the phase difference θ therebetween can be obtained. The reflection coefficient calculation unit 21 calculates the reflection coefficient Γ by obtaining the amplitude ratio r and the phase difference θ based on the travelling wave Pf and the reflected wave Pr. Specifically, the travelling wave Pf and the reflected wave Pr are transformed to a frequency domain by FFT (Fast Fourier Transform), and the amplitude ratio r and the phase difference θ are calculated by comparing the amplitudes and the phases of the travelling wave Pf and the reflected wave Pr at a frequency equal to that of the high frequency power outputted from the RF generator 2.
A capacitance calculation unit 122 calculates a capacitance of a capacitor which makes the reflection coefficient Γ close to zero based on the reflection coefficient Γ calculated by the reflection coefficient calculation unit 21. The calculation of the capacitance of the capacitor will be described later. A capacitance setting unit 23 sets and changes a capacitance of a variable capacitance capacitor in the matching circuit 30 based on the capacitance of the capacitor which is calculated by the capacitance calculation unit 122.
FIG. 2 is a block diagram of the matching circuit 30. A circuit configuration of the matching circuit 30 is determined by a variation range of the input impedance of the plasma processing apparatus 3 which acts as a load. In that case, a π-type matching circuit will be described as an example. The matching circuit 30 includes variable capacitance capacitors 31 and 32, an inductance 33, and transmission lines 35 and 36. The transmission lines 35 and 36 may be configured as coaxial cables, metal plates or the like and may include a lumped constant circuit of an inductor or a capacitor.
An input terminal 30a of the matching circuit 30 and one end of the variable capacitance capacitor 31 are connected through the transmission line 35. The other end of the variable capacitance capacitor 31 is grounded. An output terminal 30b of the matching circuit 30 and one end of the variable capacitance capacitor 32 are connected through the transmission line 36. The other end of the variable capacitance capacitor 32 is grounded.
The variable capacitance capacitors 31 and 32 and the inductance 33 serve as matching elements for matching an impedance between the RF generator 2 and the plasma processing apparatus 3. The matching circuit 30 further includes a variable capacitance capacitor control terminal 31a for controlling a capacitance of the variable capacitance capacitor 31 and a variable capacitance capacitor control terminal 32a for controlling a capacitance of the variable capacitance capacitor 32.
The variable capacitance capacitor of the matching circuit is controlled such that the reflection coefficient Γ calculated from the travelling wave Pf and the reflected wave Pr detected by the directional coupler 11 becomes close to zero. The variable capacitance at this time is calculated by the following Eqs. (2) and (3). VC1 indicates a capacitance of the variable capacitance capacitor 31. VC2 indicates a capacitance of the variable capacitance capacitor 32.VC1(n)=VC1(n−1)+real(Γ(n))*S1  Eq. (2)VC2(n)=VC2(n−1)−imag(Γ(n))*S2  Eq. (3)
Here, real( ) indicates a real part of a complex number in parentheses, and imag( ) indicates an imaginary part of a complex number in parentheses. S1 and S2 indicate coefficient and determine an update amount of the capacitance of the capacitor.
The above Eq. (2) is used for updating VC1. The above Eq. (3) is used for updating VC2. VC1(n) is calculated by adding a value obtained by multiplying the real part of the reflection coefficient Γ by the coefficient S1 to a previously updated VC1(n−1). VC2(n) is calculated by subtracting a value obtained by multiplying the imaginary part of the reflection coefficient Γ by the coefficient S2 from a previously updated VC2(n−1). Here, in VC1 and VC2, whether the update amount (real(Γ)*S1 or imag(Γ)*S2) is added or subtracted depends on the circuit type of the matching circuit 30 and the input impedance of the load to be matched.
The algorithm of such a conventional technique is disadvantageous in that VC1 and VC2 do not converge on a matching point when the load impedance changes. As described above, the impedance of the plasma load changes. The impedance of the plasma load changes abruptly before and after the ignition of the plasma and also changes depending on types, flow rates, pressures and temperatures of gases to be supplied to the plasma processing apparatus.
VC1 and VC2 for allowing the input impedance of the matching circuit 30 to be matched to 50Ω (i.e., for making the reflection coefficient zero) are determined by the load impedance connected to the output of the matching device 100. Since, however, the plasma load changes, VC1 and VC2 for the matching also change. Therefore, in the case of using the algorithm of the above Eqs. (2) and (3), VC1 and VC2 may not converge on the matching point. This is because VC1 is calculated by the above Eq. (2) and VC2 is calculated by the above Eq. (3). In other words, VC1 is calculated from the real part of the reflection coefficient and VC2 is calculated from the imaginary part of the reflection coefficient. However, the relations in the above Eqs. (2) and (3) may not be satisfied depending on the impedance of the plasma load or the capacitance of the variable capacitance capacitor. In that case, VC1 and VC2 do not converge on the matching point.    Patent document 1: PCT Publication No. WO2013/132591